The Mt Wellington firefighter said most of the guys from his flat have children now and struggle to find the time to train, but for him it's a competitive inch he just can't shake.
"I'm a competitive guy I guess. Over the years I've found myself getting more competitive with fundraising as well."
Harrison said when he started he didn't know much about leukaemia but over the years LBC had been really good at keeping participants informed with the latest statistics and research.
"It's a horrible disease with little control. It feels good to be able to take control in this and give back, even it's only in a small way."
Every day six New Zealanders are diagnosed with a blood cancer, adding up to about 2200 Kiwis a year.
Despite winning the title year after year, Harrison said it's not easy.
"You need endurance and I'd say at least 12 weeks of training. I'm not has fit as I used to be but feeling better than last year - I'm going all out."
The firefighter said he usually enters the Iron Man in March so this challenge works out well timing-wise with training.
"You need endurance and strength built up over a long time. When I started I was playing rugby and used to high-intensity, short, sharp training, but I soon discover that doesn't get you very far in this competition."
He trains with weights on a treadmill with a steady incline and long, endurance-building bike rides.
"It sounds cliched but pacing yourself is key so it's pretty critical to have that endurance-based training if your in it to win it."
About 200 brigades from all over New Zealand have been taking part in the fundraising.
So far this year firefighters have raised more than $770,000 towards a fundraising target of $1 million.
LBC general manager Georgie Hackett said the support from brigades and their local communities was humbling.
"We are so grateful for the tremendous generosity and hard work of firefighters participating in the Firefighter Sky Tower Stair Challenge.
"This fundraising is the backbone of our service and helps us grow our vital support for families living with blood cancer in their communities right across the country."
Brad Burnett from Skycity Hotels and Sky Tower said they were proud to have LBC as a charity partner.
"This incredibly successful fundraising event has raised nearly $6 million over the past 13 years."
To find out how to support your local brigade and check the current fundraising total visit www.firefightersclimb.org.nz
